1. Field of the Invention
The present invention generally relates to a circuit and a method for calibrating direct current (DC) offset.
2. Description of Related Art
The wireless communication products getting popularized in recent years mostly require miniaturization and having multiple modes in design. To meet the above-mentioned requirements, the traditional superheterodyne system used as the radio frequency (RF) component of the products has been gradually replaced by a direct conversion system or a low intermediate frequency (low IF) system, and such architecture has become the design tendency today. The reason why a direct conversion system is broadly adopted lies in that the RF signals can be directly transformed into baseband signals without the need of intermediate frequency (IF) signal processing. The complexity of the direct conversion system is much simpler than that of the superheterodyne system since the procedure of intermediate frequency (IF) signal processing is omitted. As a result, the chip can be designed in a compact size so as to have multiple modes.
In a receiver employing the direct conversion system, the major direct-current (DC) offset is generated by self-mixing of a local oscillator and the input signal of the receiver, wherein the DC offset component is determined according to the energy and frequency of the local oscillator and the input signal. Another reason lies in the mismatch of the load of an employed mixer in the process. Moreover, the DC offset may be generated because of the mismatch of the employed amplifier and filter in the process, where the DC offset component is varied with the cutoff frequency of the filter and the gain of the gain amplifier.
In the design of a conventional circuit, a high pass filter (HPF) is disposed on the signal path, so as to remove the DC offset generated thereon. FIG. 1 is a diagram showing a conventional circuit for calibrating DC offset according to U.S. Pat. No. 6,968,172 B2, wherein although the structure of using HPF 110 and 130 is able to remove the DC offset component on the signal path of an amplifier 120, but the useful low-frequency signal is removed as well.
FIG. 2 is a diagram showing a conventional circuit for calibrating DC offset according to U.S. Pat. No. 6,968,172 B2. Referring to FIG. 2, the DC offset in the signal output from an amplifier 210 is fed back to the amplifier 210 through a comparator 220 and a low pass filter (LPF) 230 so as to compensate the DC offset of the output signal. In fact, the circuit can be considered as an HPF, which means a low-frequency signal would be filtered when passing through the circuit, and the signal is required to be fed back such that the DC offset component of the signal would be compensated. Accordingly, the response speed of the circuit for the signal is reduced.
FIG. 3 is a diagram showing a conventional circuit for calibrating DC offset according to U.S. Pat. No. 6,968,172 B2. Referring to FIG. 3, the circuit depicted herein adopts one of the most common means for calibrating the DC offset in a direct conversion communication system, wherein a comparator 320 is used to detect a DC offset component, and an analog-to-digital converter (ADC) is used to convert the DC offset component into a digital signal, which is then sent to a digital signal processor (DSP) 340 for processing. After the processing of the DSP 340, the DC offset component is obtained and converted into an analog signal by a digital-to-analog converter (DAC) 350 for compensating the DC offset component of the amplifier 310.